This invention relates to a solid electrolytic capacitor using an organic semiconductor as an electrolyte. More particularly, the invention relates to a solid electrolytic capacitor in which a polymer of a heterocyclic compound such as pyrrole, furan or thiophene is used as a solid electrolyte, and to a method of manufacturing this capacitor.
Conventionally, solid electrolytic capacitors use manganese dioxide (MnO.sub.2) or 7,7,8,8-tetracyanoquinodimethane (TCNQ) salt as a solid electrolyte.
When MnO.sub.2 is employed as the solid electrolyte, a layer of MnO.sub.2 is formed on an anodic oxide film formed on the body of an anode. Ordinarily, the manufacturing method adopted as a method of forming the MnO.sub.2 layer is as follows:
(1) the anodic oxide film formed on the surface of the anode body is impregnated with manganese nitrate;
(2) the MnO.sub.2 layer is formed on the anodic oxide film by thermal decomposition;
(3) reformation is carried out; and
(4) the aforementioned steps (1) through (3) are repeated three to five times to form the MnO.sub.2 layer on the anodic oxide film of the anode body.
When a salt of TCNQ is employed as the solid electrolyte, a layer of TCNQ salt is formed on an anodic oxide film formed on the body of an anode. Ordinarily, the manufacturing method adopted as a method of forming the TCNQ salt layer is as follows: the TCNQ salt is liquified by being melted at 110.degree.-150.degree. C. if the salt is e.g. quinolinium di-(tetracyanoquinodimethane) salt and at 280.degree.-290.degree. C. if the salt is N-n-propylisoquinoline tetracyanoquinodimethane salt, the anode body is impregnated with the molten solution of TCNQ salt, and the result is cooled rapidly to form the layer of TCNQ salt on the anodic oxide film of the anode body.
Both the method of forming the MnO.sub.2 layer and that for forming the TCNQ salt layer are disadvantageous in that they involve very complicated and costly manufacturing processes, the specific resistance or ESR of the capacitors is high and there is a large leakage current. Accordingly, in order to improve upon the aforementioned problems relating to these manufacturing processes and capacitor characteristics, a novel solid electrolytic capacitor has been disclosed in which a polymer layer of a heterocyclic compound is used as the solid electrolyte. For example, see the specification of Japanese Patent Application Laid-Open Publication (KOKAI) No. 61-2315.
A solid electrolytic capacitor of this type in which the polymer layer of a heterocyclic compound is used as the solid electrolyte has a capacitor element constructed as follows.
Specifically, a dielectric oxide layer such as aluminum oxide (Al.sub.2 O.sub.3) is formed on the surface of a metal plate on which the dielectric oxide film or the like is capable of being formed. A polymer layer of a heterocyclic compound is formed on the dielectric oxide layer by electrolytic oxidation polymerization in an electrolyte solution in which a heterocyclic compound such as pyrrole, furan or thiophene has been dissolved. The polymer layer serves as the solid electrolyte. A conductive layer for electrode extraction is formed on the polymer layer, and terminals are attached to the metal plate and conductive layer, thereby fabricating the capacitor element.
However, problems arise even in this conventional solid electrolytic capacitor using the polymer layer of a heterocyclic compound as the solid electrolyte. Specifically, when the polymer layer of the heterocyclic compound is formed on the dielectric oxide layer in the process for manufacturing the capacitor element, the dielectric oxide layer is caused to deteriorate, the ability of the capacitor to withstand voltage declines and the amount of leakage current increases. The reasons are as follows:
Specifically, in the process through which the polymer layer of the heterocyclic compound is formed on the dielectric oxidation layer of the metal plate, the dielectric oxidation layer is deteriorated by the electric current during polymerization, as a result of which the amount of leakage current and dielectric loss increase, differences occur from one manufactured product to another, and yield declines due to a deterioration in insulation. Thus, many problems are encountered in terms of manufacturing uniform products economically. Consequently, the state of the art is such that the above-described solid electrolytic capacitor has yet to be realized as a commericial product and marketed.